In the production of metallic aluminum, reduction-grade Al.sub.2 O.sub.3 typically undergoes electrolysis in a Hall-Heroult cell. This cell is a large, flat-bottomed vessel, or pot, that contains a molten batch of a fluoride electrolyte, such as cryolite (Na.sub.3 AlF.sub.6), alumina (Al.sub.2 O.sub.3), and aluminum fluoride (AlF.sub.3). The pots are steel shells which are lined with a carbonaceous material. This carbonaceous liner acts as the cathode for the electrolytic process, while a carbon block suspended above the bath surface acts as the anode. During the reduction process, a high level of current passing from the anode to the cathode generates the heat required to maintain high process temperatures and to reduce electrolytically the electrolyte solution into molten aluminum metal. The molten aluminum is collected at the cathode potliner. During the life of the cathode, the carbon material becomes saturated with the cryolite electrolyte solution causing degradation of the lining. Due to the high temperatures employed in the electrolytic reduction process and chemical reactions with the molten bath, gradual aging of the carbonaceous cathode and refractory lining also takes place. The combined result of this degradation and aging is the formation of cracks in the lining, which prohibits further operation of the cell until the cracked carbonaceous lining is replaced. The unusable lining, containing primarily carbonaceous material, refractory, cryolite salt, and cyanide, is typically referred to as "spent potliner" or "spent potlining". In most instances, the spent potlining is either landfilled or stockpiled on site as a tailings pile or in sludgeponds. In large aluminum reduction facilities, spent potlining must be continuously replaced, and, consequently, the quantity of spent potlining landfilled or stockpiled increases daily.
The Environmental Protection Agency has classified spent potliner as a hazardous waste, because: (1) iron cyanide complexes, free fluorines, and free cyanides are present in significant concentrations; (2) free cyanide is extremely toxic to both humans and aquatic life if ingested; and (3) data indicates significant amounts of free cyanides, free fluorines, and iron cyanide will leach from spent potliners if they are exposed to rainwater when stored or disposed of in unprotected piles outdoors, thereby contaminating water supplies and other valuable resources. Under the EPA program, therefore, aluminum companies must treat the spent potliners prior to landfilling.
There are a number of processes directed to removing fluorines from spent aluminum potlining so that the solid waste material is more suitable for disposal in a landfill.
Several methods already have been recommended for the recovery of fluoride salts and alumina content of the spent potlining material. One method involves the pyrohydrolysis of the spent cell material or potlining in a fluidized bed reactor. Pyrohydrolysis involves contacting a fluidized bed of spent potlining with water or steam to react with the fluorine compounds to form hydrogen fluoride. During pyrohydrolysis, sodium fluoride and other sodium compounds in the potlining form sodium fluoride and sodium oxide vapors.
It is known to use limestone, calcium carbonate, to react with fluoride in the spent potlining at about 700.degree.-780.degree. C. to form a calcium fluoride. However, the final product still has a high level of leachable fluoride. An article in Light Metals, 1981, entitled "Recovery of Fluoride and Fluorine Balance of Aluminum Smelting Plant" describes a process of treating spent potlining wherein the potlining is crushed, incinerated, and introduced into a crusher for pulverizing and extraction of the fluorides into water. After filtration, the material is introduced into a mixing tank, and sulfuric acid and calcium carbonate are added to produce calcium fluoride. The slurry is desiccated, and the sludge sent to a disposal site. The fluoride-containing filtrate is used as the scrubbing solution in a wet scrubber.
U.S. Pat. No. 4,113,832 to Bell et. al. discloses a process in which spent potlining undergoes pyrohydrolysis, substantially combusting all of the carbon content and removing the fluoridic components of the spent potlining with off-gases. The process produces a solid clinker-type material that is subjected to alkaline digestion for recovery of an alkali aluminate solution from which high purity aluminum is recovered.
U.S. Pat. No. 4,444,740 to Snodgrass et. al. discloses a process to treat spent potlining by incinerating it at a temperature suitable to destroy the cyanides and substantially to limit volatilization of the fluorides. This is followed by leaching the incineration step's solid residue with a dilute caustic. The leachate is treated with a calcium compound to precipitate calcium fluoride. The calcium fluoride is ultimately used to recover fluorides which serve as a feed material to a reduction cell for aluminum manufacture. The solids resulting from this process are sent to a disposal site.
U.S. Pat. No. 4,735,784 to Davis et. al. melts spent potlining into a liquid slag by combusting carbon present in the spent potliner in a smeltor at a temperature of 1000.degree.-1700.degree. C. The slag is then held at elevated temperatures for a period of time sufficient for the contaminants to decompose to harmless components or for a portion of the contaminants to evaporate or sublime from the liquid. The liquid is then cooled, whereby a glassy solid is formed with contaminant residues encapsulated in the slag. The glassy solid is suitable for landfill disposal. Steam is added to the smeltor to convert fluorides in the spent potliner to hydrogen fluoride gas. Additionally, fluoride gas is generated from slag in the holding furnace. As a result, substantially all fluorides are volatilized, with any remaining fluorides in the glassy solid being fully immobilized.
U.S. Pat. No. 5,024,822 to Hittner et. al. discloses a process for treating spent potlining by incineration at a temperature low enough to maintain low fluorine vapor pressure and high enough to combust at least 50% by weight of the carbonaceous material in the potlining, typically 500.degree.-850.degree. C. Vaporization of fluorine is limited by combusting the carbonaceous material at a low moisture content. A siliceous material is admixed with the potlining either before or after this ash forming stage. The ash and siliceous material are heated to a temperature of at least 1100.degree. C. to form a glassy residue in which fluorides are dispersed and fixed. In view of the low leachability of fluoride from this material, it is suitable for landfill.
Although these procedures have been found to be useful in converting spent aluminum potlining to a form suitable for landfill, they do not produce a useful product. As a result, economic justification for the capital and operational costs of implementing such procedures for treatment of spent potlining tend to be problematic. The present invention is directed to overcoming these deficiencies.